1. Field of the Invention
The present invention relates to a cutting tool and, in particular, a cutting tool comprising a cemented carbide which has excellent plastic deformation resistance, high strength, and excellent wear resistance.
2. Description of Related Art
As the cemented carbide conventionally used widely for cutting of metals, there is known, for example, a WC—Co alloy made up of a hard phase composed mainly of WC (tungsten carbide), and a binder phase composed of an iron family metal such as Co (cobalt), or the system in which a solid solution phase such as carbide, nitride, carbon nitride, or the like of metals of the groups IV, V, and VI in the periodic table is dispersed in the above-mentioned WC—Co alloy. These cemented carbides are mainly used for cutting of carbon steel, alloy steel, or the like.
In recent years, forgings of further complicated shape have been often used because of the improved forging technique and the rapid advancement of near net shaping of work material. The work material having the complicated shape often include an interrupted cutting part. Additionally, high efficient cutting is required and high speed cutting is advanced in order to reduce machining costs. Hence, there is the need for a cutting tool that can allow for high-speed and strong interrupted cutting.
For example, Japanese Unexamined Patent Publication No. 4-293749 discloses a cemented carbide consisting of 4 to 20 weight % of Co, 0.2 to 20 weight % of carbide of transition metals of the groups IV, V, and VI in the periodic table, except for W (tungsten), WC, and unavoidable impurities. There is also disclosed that the fracture resistance in interrupted cutting and the wear resistance in continuous cutting can be improved by reducing the nitrogen content of the cemented carbide to 0.005 to 0.200 weight %, and reducing the oxygen content to 0.001 to 0.200 weight %.
In the cemented carbide obtained by simply reducing oxygen amount and nitrogen amount in the entire sintered body, as in the case with the above publication, transverse rupture force can be improved thereby to improve the fracture resistance when used for interrupted cutting and the wear resistance when used for continuous cutting. However, its plastic deformation resistance is insufficient for cutting such as high-speed interrupted cutting where a cutting edge is heated and subjected to a strong impact. Therefore, the cutting edge will be deformed, so that cutting accuracy is lowered and the cutting surface is rough.
Japanese Unexamined Patent Publication No. 11-36022 discloses the method of sintering by adding, as raw material powder for manufacturing a cemented carbide containing a plate crystal WC, a tungsten powder, a binder phase consisted powder of such as cobalt or the like, a carbon powder, and an oxygen-containing compound powder composed of at least one of oxide, carboxide, nitroxide, oxycarbonitride of metals of the groups IV, V, and VI in the periodic table, and solid solution of these. In accordance with this method, the oxygen-containing compound powder in the raw material is firstly reacted with carbon, thereby generating carbon oxide. When this is heated, the oxygen of the carbon oxide is gradually reacted with carbon and changed to carbide. On the other hand, the tungsten powder consists a complex carbide together with carbon and cobalt or nickel, and the complex carbide exists stably up to high temperature because the above-mentioned oxygen-containing compound consumes carbon, and hence no carbon is supplied thereto. There is disclosed that a large amount of plate crystal WC having a high aspect ratio are deposited from a large amount of the complex carbide.
In the cemented carbide wherein the plate crystal WC is allowed to deposit by adding the oxygen-containing compound powder into the raw material powder, as in the above publication, hardness and strength can be improved at the same time by the presence of the plate crystal WC. However, the oxygen amount remaining at a B1-type solid solution phase in the sintered body cannot be controlled, so that a large amount of oxygen remain and the hardness and strength of the B1-type solid solution phase are lowered. These are insufficient for satisfying the plastic deformation resistance required for high-speed interrupted cutting.
Japanese Unexamined Patent Publication No. 11-335769 discloses the method of manufacturing a cemented carbide by using, as raw material powders, a tungsten carbide powder (raw material A) having a particle size of 0.6 to 1 μm, a tungsten carbide powder (raw material B) whose particle size is not less than two times that of the raw material A, a binder phase consisted metal powder (raw material C) such as metal cobalt, and at least one of carbide, nitride, oxide (except for tungsten carbide) powder (raw material D) selected from elements of the groups IV, V, and VI in the periodic table, or a solid solution phase of these, having a mean particle size of 0.01 to 0.5 μm. There is disclosed that when forming and sintering are carried out by adding a raw material powder containing oxygen as being essential for the raw material D, it is possible to make plate WC particles containing, in the interior of crystal particles thereof, a compound composed of at least one of oxide, carboxide, nitroxide, and oxycarbonitride selected from elements of the groups IV, V, and VI in the periodic table, or a solid solution phase of these.
In the cemented carbide containing the compound particles containing oxygen in the interior of the plate WC, as in the case with the above publication, strain occurs in the crystal particles of the plate WC, and the strain enhances the WC crystal particles thereby to minimize variations in the strength of the cemented carbide, leading to excellent hardness and strength. However, only the enhancement of the WC crystal particles is insufficient to suppress the progress of cracks in high-speed interrupted cutting. That is, in the high-speed interrupted cutting where the cutting edge is heated, for example, in the cases where complicated-shaped forgings made of carbon steel or alloy steel, such as a knuckle and a pinion gear, are cut interruptedly at high speed, it cannot be said that the plastic deformation resistance suffices for the cutting. Therefore, in the high-speed interrupted cutting where the cutting edge is heated and subjected to a impact, the cutting edge causes plastic deformation, and the plastic deformation leads to anomalous wear and film peeling, resulting in a short tool life.